v0.21.4 — hollow shapes, told honestly

Mesh decomposes concave meshes into convex pieces now, so a cup holds what you drop into it. The bundled physics docs said the opposite — that a hollow shape can never be a container and agents should hand-build proxy walls out of primitives — which is now both wrong and worse than modelling the shape directly.

Docs only: no tool added, tool count unchanged, server untouched.